Iridium-Catalyzed Asymmetric Hydrogenation of Tetrasubstituted α

Letter Cite This: Org. Lett. 2018, 20, 6349−6353

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Iridium-Catalyzed Asymmetric Hydrogenation of Tetrasubstituted α‑Fluoro-β-enamino Esters: Efficient Access to Chiral α‑Fluoro-βamino Esters with Two Adjacent Tertiary Stereocenters Zhengyu Han,† Yu-Qing Guan,† Gang Liu,† Rui Wang,† Xuguang Yin,† Qingyang Zhao,§ Hengjiang Cong,† Xiu-Qin Dong,*,† and Xumu Zhang*,‡,†

Org. Lett. 2018.20:6349-6353. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/19/18. For personal use only.

†

Key Laboratory of Biomedical Polymers, Engineering Research Center of Organosilicon Compounds & Materials, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China ‡ Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen, Guangdong 518000, People’s Republic of China § Key Laboratory of Synthetic and Natural Functional Molecule, Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, People’s Republic of China S Supporting Information *

ABSTRACT: An iridium-catalyzed highly efficient asymmetric hydrogenation of challenging tetrasubstituted α-fluoro-βenamino esters was successfully developed using bisphosphine−thiourea (ZhaoPhos) as the ligand, which prepared a series of chiral α-fluoro-β-amino esters containing two adjacent tertiary stereocenters with high yields and excellent diastereoselectivities/enantioselectivities (73%−99% yields, >25:1 dr, 91%−>99% ee, and turnover number (TON) values up to 8600), and no defluorinate byproduct was detected.

T

hindrance.11,12 To the best of our knowledge, there is seldom an asymmetric catalytic reduction of tetrasubstituted α-fluoroβ-enamino esters to prepare chiral α-fluoro-β-amino esters.12 Since our privileged bisphosphine-thiourea (ZhaoPhos) ligand has proven to be highly efficient in Rh-catalyzed asymmetric hydrogenations of prochiral disubstituted and trisubstituted unsaturated functionalized compounds,13 we were convinced that this challenging asymmetric hydrogenation of tetrasubstituted α-fluoro-β-enamino esters could be achieved through this unique thiourea motif-assisted catalytic system. Indeed, we herein successfully realized a highly efficient Ir/bisphosphinethiourea (ZhaoPhos)-catalyzed asymmetric hydrogenation of tetrasubstituted α-fluoro-β-enamino esters; various chiral αfluoro-β-amino esters containing two adjacent tertiary stereocenters were prepared with high yields and excellent diastereoselectivities/enantioselectivities (Scheme 1; 73%− 99% yields, >25:1 diastereomeric ratio (dr), and 91%−>99% enantiomeric excess (ee), turnover number (TON) values up to 8600), and no defluorinate byproduct was detected in this catalytic system. We started our initial research of the asymmetric hydrogenation of tetrasubstituted α-fluoro-β-enamino esters using compound 1a12 as a model substrate to investigate metal precursors at room temperature. Trace conversion and messy

he organofluorine compounds always exhibit distinctive properties from their parent compounds, because of the unique characters of the F atom, including high electronegativity, small steric size, and C−F bond strength.1 The chemistry of chiral organofluorine compounds is a fastdeveloping research area and has attracted great attention, because of their wide range of applications in some important fields, such as agriculturals, pharmaceuticals, and materials.1d,2,3a,c For example, fluorination of drug molecules may modify metabolic stability, biological activity, and protein− drug interactions.1d,3 Fluorinated amino acids and peptides have wide application in bioorganic and medical fields, such as biological tracers, enzyme inhibitors, mechanistic probes.3b,4,5 Compared with other chiral fluorinated amino acids and derivatives, there are a limited number of synthetic methods for the construction of highly stereoselective chiral α-fluoro-βamino acids and derivatives.6−8 Most synthetic methodologies for the construction of chiral α-fluoro-β-amino acids and derivatives have been well-established through asymmetric αfluorination of carbonyl compounds or nucleophilic addition of fluorinated ketoesters.8 Asymmetric reduction of prochiral functionalized enamides is regarded as one of the most powerful and important approaches to chiral amine derivatives, but it is mainly restricted to disubstituted and trisubstituted enamides.9,10 It is challenging to realize the asymmetric hydrogenation of tetrasubstituted enamides, because of their unfavorable steric © 2018 American Chemical Society

Received: August 5, 2018 Published: October 2, 2018 6349

DOI: 10.1021/acs.orglett.8b02503 Org. Lett. 2018, 20, 6349−6353

Letter

Organic Letters Scheme 1. Ir-Catalyzed Asymmetric Hydrogenation of αFluoro-β-enamino Esters

Table 2. Screening Ligands for Asymmetric Hydrogenation of Ethyl (E)-2-Fluoro-3-phenyl-3-(phenylamino)acrylate 1aa

reaction system were observed using Rh(NBD)2BF4, Rh(COD)2BF4, and [Rh(COD)Cl]2 as metal precursors (Table 1, entries 1−3). Full conversion, >25:1 dr, and 98% ee were obtained in the presence of [Ir(COD)Cl]2 (Table 1, entry 4). This Ir-catalyzed asymmetric hydrogenation was then performed in different solvents to examine the effect of solvent. High conversions, excellent diastereoselectivities/ enantioselectivities can be obtained in 1,2-dichloroethane (DCE) and CHCl3 (97%−99% conversions, >25:1 dr, and 93%−96% ee; Table 1, entries 6 and 9). No desired hydrogenation products were detected when other usual solvents were tested, such as tetrahydrofuran (THF), toluene, 1,4-dioxane, MeOH (Table 1, entries 5, 7, 8, 10). When this asymmetric hydrogenation was conducted under 10 atm H2, moderate conversion was obtained (77% conversion; Table 1, entry 11). In addition, this hydrogenation can be finished within 12 h (>99% conversion, >25:1 dr, and 98% ee; Table 1, entry 12), and there is no defluorinate byproduct in the reaction system. Several bisphosphine−(thio)urea ligands were then applied into the iridium-catalyzed asymmetric hydrogenation of tetrasubstituted α-fluoro-β-enamino ester 1a. As shown in Table 2, high reactivity and excellent diastereoselectivity/ enantioselectivity can be obtained in the presence of ZhaoPhos L1 (>99% conversion, 98% ee, >25:1 dr; Table 2, entry 1). The N-methylation of ZhaoPhos L2 provided poor conversion and moderate enantioselectivity (49% conversion, 76% ee,

entry

ligand

conversionb (%)

diastereomeric ratio, drb

enantiomeric excess, eec (%)

1 2 3 4 5

ZhaoPhos L1 L2 L3 L4 L5

>99 49 25:1 >25:1 NAd NAd NAd

98 76 NAd NAd NAd

a

Reaction conditions: 0.05 mmol 1a in 1.0 mL CH2Cl2, S/C = 100, 50 atm H2, room temperature, 12 h. bDetermined by 1H NMR analysis. cDetermined by HPLC analysis using a chiral stationary phase. dNA = not available. eNR = no reaction.

>25:1 dr; Table 2, entry 2). The ligand L3 without two trifluoromethyl groups on the phenyl ring displayed very poor reactivity, and 25:1 NAd NAd >25:1 NAd >25:1 >25:1

NAd NAd NAd 98 NAd 93 NAd NAd 96 NAd 98 98

a Reaction conditions: 0.05 mmol 1a, 1.0 mL solvent, 50 atm H2, room temperature, 24 h. For entries 4−12: the catalyst was precomplexed with 0.05 mmol [Ir(COD)Cl]2 and 0.011 mmol ligand L1 in 1.0 mL CH2Cl2 (50 μL for each reaction vial). bDetermined by 1H NMR analysis. c Determined by HPLC analysis using a chiral stationary phase. dNA = not available. eNR = no reaction. f10 atm H2. gReaction time is 12 h.

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DOI: 10.1021/acs.orglett.8b02503 Org. Lett. 2018, 20, 6349−6353

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Organic Letters

results (73%−99% yields, 91%−>99% ee, >25:1 dr). The Nphenyl amine-derived α-fluoro-β-enamino ethyl ester substrates (1a−1e) with an R group that is either an electron-rich or electron-poor group proceeded well with full conversion, excellent enantioselectivities, and high diastereoselectivities (94%−98% yields, 95%−99% ee, >25:1 dr). In addition, the Ircatalyzed asymmetric hydrogenation of tetrasubstituted αfluoro-β-enamino ethyl ester substrates with different substituted groups on the phenyl ring of the aniline motif was also performed, the hydrogenation products chiral α-fluoro-βamino ethyl esters (2f−2j) were afforded with good to excellent results (80%−>99% conversions, 73%−98% yields, 91%−>99% ee, >25:1 dr). When the ethyl ester group was changed to a methyl ester group, the substrates (1k−1m) also worked well in this transformation, and excellent results were achieved (>99% conversion, 93%−99% yields, 97%−99% ee, >25:1 dr). To our delight, the Ir-catalyzed asymmetric hydrogenation of model substrate tetrasubstituted α-fluoro-β-enamino ethyl ester 1a could be performed with very low catalyst loading. When the catalyst loading was gradually decreased from 1.0 mol % (S/C = 100) to 0.2 mol % (S/C = 500) and 0.1 mol % (S/C = 1000), the hydrogenation product 2a could be still obtained with excellent results (full conversion, >25:1 dr, and 98% ee; Table 3, entries 1−3). This asymmetric hydrogenation

thiourea motif did not promote this asymmetric hydrogenation successfully (Table 2, entries 4 and 5). With the optimized reaction conditions in hand, we turned our attention to investigating the substrate generality of this Ir/ ZhaoPhos-catalyzed asymmetric hydrogenation of tetrasubstituted α-fluoro-β-enamino esters. The results are summarized in Scheme 2. A wide range of tetrasubstituted α-fluoro-βenamino ethyl esters substrates with different substituents on the phenyl ring were hydrogenated smoothly with excellent Scheme 2. Substrate Scope Study for Asymmetric Hydrogenation of Tetrasubstituted α-Fluoro-β-enamino Estersa

Table 3. TON Experiments for the Iridium-Catalyzed Asymmetric Hydrogenation of Ethyl (E)-2-Fluoro-3-phenyl3-(phenylamino)acrylate 1aa

entry

S/C

temperature (°C)

H2 (atm)

time (h)

conversionb (%)

eec (%)

1 2 3 4 5

100 500 1000 5000 10000

25 25 45 45 45

50 70 80 80 80

12 24 48 60 60

>99 >99 >99 >99 86

98 98 98 98 98

a

Unless otherwise noted, all reactions were performed with a [Ir(COD)Cl]2/ZhaoPhos L1 = 0.5:1.1 in CH2Cl2. bDetermined by 1 H NMR analysis. cDetermined by HPLC analysis using a chiral stationary phase.

also proceeded smoothly with only 0.02 mol % catalyst (S/C = 5000), full conversion, and excellent diastereoselectivity/ enantioselectivity was achieved (>99% conversion, >25:1 dr, and 98% ee; Table 3, entry 4). Good conversion and excellent diastereoselectivity/enantioselectivity was obtained even with 0.01 mol % catalyst (S/C = 10000) (86% conversion, TON = 8600, >25:1 dr, and 98% ee; Table 3, entry 5). These results indicate that our Ir/ZhaoPhos L1 catalytic system possessed high activity in this asymmetric hydrogenation. As shown in Scheme 3, it is possible that the equilibration of the enamine 1h and the imine 1h′ exists in the reaction system, and they may participate in this asymmetric hydrogenation. In order to investigate the reaction mechanism, this asymmetric hydrogenation of the enamine 1h in the presence of D2 was conducted under the optimizated reaction conditions. We found that the product was extensively deuterated at both the

a

Reaction conditions: 0.05 mmol substrate 1 in 1.0 mL CH2Cl2, S/C = 100, 50 atm H2, room temperature, 12 h. Conversion and dr was determined by 1H NMR analysis; ee was determined by HPLC analysis using a chiral stationary phase. bS/C = 50, reaction time is 36 h. cS/C = 50. 6351

DOI: 10.1021/acs.orglett.8b02503 Org. Lett. 2018, 20, 6349−6353

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Scheme 3. Deuterium-Labeling Experiment

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02503. Optimization reaction conditions; procedures; NMR and HPLC spectra (PDF) Accession Codes

CCDC 1844417 (for product 2d) and 1844690 (for substrate 1g) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

α- and β-positions. These observations suggest that the enamine 1h was involved in this asymmetric hydrogenation and the imine 1h′ was much less likely. The hydrogenation products 2 containing two adjacent tertiary stereogenic centers can be readily converted to other synthetic useful compounds, as exemplified in Scheme 4. The

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-Q. Dong). *E-mail: [email protected] (X. Zhang).

Scheme 4. Synthetic Transformations of Hydrogenation Product 2

ORCID

Hengjiang Cong: 0000-0002-9225-0095 Xumu Zhang: 0000-0001-5700-0608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 21432007 and 21502145), the Natural Science Foundation of Hubei Province (Grant No. 2016CFB449), the Wuhan Morning Light Plan of Youth Science and Technology (Grant No. 2017050304010307), Shenzhen Nobel Prize Scientists Laboratory Project (Grant No. C17213101) and the Fundamental Research Funds for the Central Universities (Grant No. 2042018kf0202). The Program of Introducing Talents of Discipline to Universities of China (111 Project) is also appreciated.

hydrogenation product 2a was efficiently reduced by lithium aluminum hydride (LiAlH4) to afford chiral β-fluoro-γ-amino alcohol 3 without loss of stereoselectivity (92% yield, >25:1 dr, 98% ee).14 Hydrolysis of compound 2a provided chiral αfluoro-β-amino acid 4 with 85% yield, >25:1 dr, and 98% ee.15 The chiral unprotected α-fluoro-β-amino ester 5 can be easily obtained from compound 2h in the presence of ceric ammonium nitrate (CAN) with 71% yield, >25:1 dr, and 99% ee.16 In summary, we successfully developed Ir/ZhaoPhoscatalyzed asymmetric hydrogenation of challenging tetrasubstituted α-fluoro-β-enamino esters. Various chiral α-fluoro-βamino esters containing two adjacent tertiary stereocenters were obtained with high yields and excellent diastereoselectivities/enantioselectivities (73%−99% yields, >25:1 dr, 91%−>99% ee). In addition, our catalytic system displayed very high activity in this challenging asymmetric hydrogenation (up to TON = 8600). Moreover, this powerful synthetic utility of hydrogenation products was performed to construct other useful chiral compounds, such as chiral β-fluoro-γ-amino alcohol, α-fluoro-β-amino acid, and unprotected α-fluoro-βamino ester in high yields and without any loss of stereoselectivity.

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DOI: 10.1021/acs.orglett.8b02503 Org. Lett. 2018, 20, 6349−6353